1. Field of the Invention
The present invention relates to an ultrasonic coordinate input apparatus for determining input coordinates by utilizing the propagation of ultrasonic wave generated by ultrasonic input means such as ultrasonic pen.
2. Description of Related Art
As the computer technology advances, apparatuses based on pen input method, wherein handwritten letters and diagrams are inputted to a processing apparatus such as a computer by means of a pen-like device, are attracting much attention as new man-machine interfaces in recent years. The pen input apparatus has a display device and a coordinate detection device being combined, whereas letter or the like is inputted by a pen on the display screen of the display device and the coordinates of the pen are detected by the coordinate detection device so that various information is displayed on the display device according to the result of detection.
Major methods known at present, for the coordinate detection device for the pen input apparatus are electromagnetic induction method, resistive membrane method and ultrasonic wave method. With the electromagnetic induction method, because an opaque coordinate sensing panel with a special receiving coil pattern disposed thereon is used as a receiving device, it is necessary to install the panel on back of the display device. With the resistive membrane method, because a resistive membrane glass panel having been subjected to a special process to form a transparent conductive thin film over the entire surface thereof is used as the coordinate detection unit, transmittance for the displaying light decreases to make the displayed items difficult to recognize for the eye and the electricity consumed for display increases.
For the reasons described above, such ultrasonic coordinate input, apparatuses are widely used as ultrasonic wave is emitted by an ultrasonic pen provided with an ultrasonic wave generating element and is propagated through an ultrasonic propagating medium, while the propagated ultrasonic wave is detected by a plurality of piezoelectric oscillators installed on the ultrasonic propagating medium, thereby to determine the coordinates of the ultrasonic pen on the ultrasonic propagating medium.
FIG. 1 is a schematic diagram showing the basic configuration of the ultrasonic coordinate input apparatus as described above. FIGS. 2A, B are drawings to explain the principle of detecting the coordinates in the ultrasonic coordinate input apparatus. In the drawings, numeral 41 denotes an ultrasonic pen having a piezoelectric oscillator for oscillating ultrasonic waves, while the ultrasonic pen 41 is in contact with the ultrasonic propagating medium 42 that propagates ultrasonic waves. Installed on the periphery of the ultrasonic propagating medium 42 are a plurality of (three in this example) receiving sensors (receiving piezoelectric oscillators) 43, that receive the ultrasonic wave propagated through the ultrasonic propagating medium 42, being bonded with a conductive adhesive. Ultrasonic wave emitted by the ultrasonic wave input pen 41 propagates as Lamb wave in the ultrasonic propagating medium 42 and is received by the receiving sensors 43. An ultrasonic wave detection circuit 44 including a pre-amplifier and other components detects the ultrasonic wave received by the receiving sensors 43 and sends the detection signal to an arithmetic circuit 45. The arithmetic circuit 45 carries out arithmetic operations to be described later on the detection signal received from the ultrasonic wave detection circuit 44, thereby to determine the coordinates. The coordinates determined are displayed on a display 46.
The principle of detecting the coordinates will be described below. As shown in FIGS. 2A, B, the coordinates can be determined on the basis of the time required for the ultrasonic wave emitted by the ultrasonic pen 41 to propagate in the ultrasonic propagating medium 42 and reach the two receiving sensors 43 and to be received thereby. Assume that ultrasonic wave is emitted by the ultrasonic pen 41 at a point P(x, y), the two receiving sensors 43, 43 are located at A(0, 0) and B(L, 0), the ultrasonic wave propagates in the ultrasonic propagating medium 42 at a velocity C, time required to propagate between the points P and A is t.sub.1, and time required to propagate between the points P and B is t.sub.2, then the coordinates of the point P can be determined as follows. Quantities L and C are knows and t.sub.1 and t.sub.2 are measured values. EQU (C.t.sub.1).sup.2 =x.sup.2 +y.sup.2 ( 1) EQU (c.t.sub.2).sup.2 =(L-x).sup.2 +y.sup.2 ( 2)
Subtracting (1) from (2) yields the following. EQU x={C.sup.2 (t.sub.1.sup.2 -t.sub.2.sup.2)+L.sup.2 }/2L (3)
From (1) and (3) the following equation is derived. EQU y=[C.sup.2.t.sub.1.sup.2 -{L.sup.2 +C.sup.2 (t.sub.1.sup.2 -t.sub.2.sup.2)}.sup.2 /4L.sup.2 ].sup.1/2 ( 4)
FIG. 3 is a drawing showing the configuration of an ultrasonic coordinate input apparatus of the prior art disclosed in Japanese Patent Application Laid-Open No. 1-126716(1989). In FIG. 3, components identical with those shown in FIG. 1 are denoted by the same reference numerals and a drawing of the ultrasonic pen is omitted. In this example, the pre-amplifier 47 is not incorporated in the ultrasonic wave detection circuit 44 but is separated therefrom, and the three pre-amplifiers 47 are installed in a propagating medium mounting member 48 provided for mounting the ultrasonic propagating medium 42 on the display 46 and are connected to the corresponding receiving sensors 43 by means of wires 49 about several tens of millimeters long.
FIG. 4 is a drawing showing the configuration of an ultrasonic coordinate input, apparatus of the prior art disclosed in the Japanese Patent Application Laid-Open No. 1-126716 (1989) described above as another embodiment. The ultrasonic propagating medium 42 is placed on the propagating medium mounting member 48, and a printed circuit board 50, whereon three receiving sensors 43 and three pre-amplifiers 47 each connected to corresponding one of the receiving sensors 43 by means of a wire 49 about several tens of millimeters long are formed, is installed on the ultrasonic propagating medium 42.
In an ultrasonic coordinate input apparatus of an ultrasonic wave type, because the coordinates are determined by measuring the delay time required for the ultrasonic wave signal emitted by the ultrasonic pen 41 to propagate in the ultrasonic propagating medium 42 and to reach the receiving sensor 43, it is necessary to input the time when the ultrasonic pen 41 emits the ultrasonic wave to the arithmetic circuit 45 as the reference time for measuring the delay time. Therefore, the ultrasonic pen 41 must be connected to the main unit with a cord 51, resulting in inconvenience as the cord 51 causes hindrance to the operation in the case of the coordinate input apparatus of the prior art.
In the configuration shown in FIG. 3, because the pre-amplifier 47 and other components are mounted on the propagating medium mounting member 48 located on the periphery of the ultrasonic propagating medium 42 and the pre-amplifiers 47 and the receiving sensors 43 are connected with the wires 49, a large-sized propagating medium mounting member 48 is required to install the ultrasonic propagating medium 42, resulting in a problem that the entire apparatus becomes large in size.
The configuration shown in FIG. 4 has a problem that the display is obscured by the printed circuit board 50 when the ultrasonic coordinate input apparatus is installed on the display. Although a transparent printed circuit board may be used with a wiring pattern being formed outside the display section of the display and the pre-amplifiers and other components installed outside the display section, such problems remain as increased parallax and decreasing transmittance.